#include "sofa.h"
#include "sofam.h"

int iauTdbtcb(double tdb1, double tdb2, double *tcb1, double *tcb2)
/*
**  - - - - - - - - - -
**   i a u T d b t c b
**  - - - - - - - - - -
**
**  Time scale transformation:  Barycentric Dynamical Time, TDB, to
**  Barycentric Coordinate Time, TCB.
**
**  This function is part of the International Astronomical Union's
**  SOFA (Standards of Fundamental Astronomy) software collection.
**
**  Status:  canonical.
**
**  Given:
**     tdb1,tdb2  double    TDB as a 2-part Julian Date
**
**  Returned:
**     tcb1,tcb2  double    TCB as a 2-part Julian Date
**
**  Returned (function value):
**                int       status:  0 = OK
**
**  Notes:
**
**  1) tdb1+tdb2 is Julian Date, apportioned in any convenient way
**     between the two arguments, for example where tdb1 is the Julian
**     Day Number and tdb2 is the fraction of a day.  The returned
**     tcb1,tcb2 follow suit.
**
**  2) The 2006 IAU General Assembly introduced a conventional linear
**     transformation between TDB and TCB.  This transformation
**     compensates for the drift between TCB and terrestrial time TT,
**     and keeps TDB approximately centered on TT.  Because the
**     relationship between TT and TCB depends on the adopted solar
**     system ephemeris, the degree of alignment between TDB and TT over
**     long intervals will vary according to which ephemeris is used.
**     Former definitions of TDB attempted to avoid this problem by
**     stipulating that TDB and TT should differ only by periodic
**     effects.  This is a good description of the nature of the
**     relationship but eluded precise mathematical formulation.  The
**     conventional linear relationship adopted in 2006 sidestepped
**     these difficulties whilst delivering a TDB that in practice was
**     consistent with values before that date.
**
**  3) TDB is essentially the same as Teph, the time argument for the
**     JPL solar system ephemerides.
**
**  Reference:
**
**     IAU 2006 Resolution B3
**
**  This revision:  2021 May 11
**
**  SOFA release 2021-05-12
**
**  Copyright (C) 2021 IAU SOFA Board.  See notes at end.
*/
{

/* 1977 Jan 1 00:00:32.184 TT, as two-part JD */
   static const double t77td = DJM0 + DJM77;
   static const double t77tf = TTMTAI/DAYSEC;

/* TDB (days) at TAI 1977 Jan 1.0 */
   static const double tdb0 = TDB0/DAYSEC;

/* TDB to TCB rate */
   static const double elbb = ELB/(1.0-ELB);

   double d, f;


/* Result, preserving date format but safeguarding precision. */
   if ( fabs(tdb1) > fabs(tdb2) ) {
      d = t77td - tdb1;
      f  = tdb2 - tdb0;
      *tcb1 = tdb1;
      *tcb2 = f - ( d - ( f - t77tf ) ) * elbb;
   } else {
      d = t77td - tdb2;
      f  = tdb1 - tdb0;
      *tcb1 = f - ( d - ( f - t77tf ) ) * elbb;
      *tcb2 = tdb2;
   }

/* Status (always OK). */
   return 0;

/* Finished. */

/*----------------------------------------------------------------------
**
**  Copyright (C) 2021
**  Standards Of Fundamental Astronomy Board
**  of the International Astronomical Union.
**
**  =====================
**  SOFA Software License
**  =====================
**
**  NOTICE TO USER:
**
**  BY USING THIS SOFTWARE YOU ACCEPT THE FOLLOWING SIX TERMS AND
**  CONDITIONS WHICH APPLY TO ITS USE.
**
**  1. The Software is owned by the IAU SOFA Board ("SOFA").
**
**  2. Permission is granted to anyone to use the SOFA software for any
**     purpose, including commercial applications, free of charge and
**     without payment of royalties, subject to the conditions and
**     restrictions listed below.
**
**  3. You (the user) may copy and distribute SOFA source code to others,
**     and use and adapt its code and algorithms in your own software,
**     on a world-wide, royalty-free basis.  That portion of your
**     distribution that does not consist of intact and unchanged copies
**     of SOFA source code files is a "derived work" that must comply
**     with the following requirements:
**
**     a) Your work shall be marked or carry a statement that it
**        (i) uses routines and computations derived by you from
**        software provided by SOFA under license to you; and
**        (ii) does not itself constitute software provided by and/or
**        endorsed by SOFA.
**
**     b) The source code of your derived work must contain descriptions
**        of how the derived work is based upon, contains and/or differs
**        from the original SOFA software.
**
**     c) The names of all routines in your derived work shall not
**        include the prefix "iau" or "sofa" or trivial modifications
**        thereof such as changes of case.
**
**     d) The origin of the SOFA components of your derived work must
**        not be misrepresented;  you must not claim that you wrote the
**        original software, nor file a patent application for SOFA
**        software or algorithms embedded in the SOFA software.
**
**     e) These requirements must be reproduced intact in any source
**        distribution and shall apply to anyone to whom you have
**        granted a further right to modify the source code of your
**        derived work.
**
**     Note that, as originally distributed, the SOFA software is
**     intended to be a definitive implementation of the IAU standards,
**     and consequently third-party modifications are discouraged.  All
**     variations, no matter how minor, must be explicitly marked as
**     such, as explained above.
**
**  4. You shall not cause the SOFA software to be brought into
**     disrepute, either by misuse, or use for inappropriate tasks, or
**     by inappropriate modification.
**
**  5. The SOFA software is provided "as is" and SOFA makes no warranty
**     as to its use or performance.   SOFA does not and cannot warrant
**     the performance or results which the user may obtain by using the
**     SOFA software.  SOFA makes no warranties, express or implied, as
**     to non-infringement of third party rights, merchantability, or
**     fitness for any particular purpose.  In no event will SOFA be
**     liable to the user for any consequential, incidental, or special
**     damages, including any lost profits or lost savings, even if a
**     SOFA representative has been advised of such damages, or for any
**     claim by any third party.
**
**  6. The provision of any version of the SOFA software under the terms
**     and conditions specified herein does not imply that future
**     versions will also be made available under the same terms and
**     conditions.
*
**  In any published work or commercial product which uses the SOFA
**  software directly, acknowledgement (see www.iausofa.org) is
**  appreciated.
**
**  Correspondence concerning SOFA software should be addressed as
**  follows:
**
**      By email:  sofa@ukho.gov.uk
**      By post:   IAU SOFA Center
**                 HM Nautical Almanac Office
**                 UK Hydrographic Office
**                 Admiralty Way, Taunton
**                 Somerset, TA1 2DN
**                 United Kingdom
**
**--------------------------------------------------------------------*/
}